1. Field of the Invention
This invention generally relates to an insulated conduit having particular utility in subterranean wells and more particularly to a concentric walled insulated conduit having an annular space between the walls within which is deposited an insulating material and sealed therein.
2. Description of the Prior Art
In producing some subterranean wells, steam is injected into an injection well to increase recovery of hydrocarbons and for reducing high viscosity crude oil, otherwise known as "heavy crude", to a viscosity making it more readily pumpable. One technique for doing this is to inject a high quantity of steam into the production zone containing "heavy crude" for an extended period of time, such as from about three to about five weeks, at which point the viscosity of the heated crude will be reduced, and, thus made readily pumpable, through a production well in communication with the production zone, or by modifying the injection well. A steam "flood" may also be provided by known techniques, generally through an injection well, to drive the flood and the produced hydrocarbons to a nearby production well.
One of the major problems in injecting steam into a subterranean production zone through conventional well production tubing is that the steam loses a large quantity of its heat to the well bore casing and surrounding formation as it travels downwardly to the production zone.
Attempts have been made in the past to reduce the heat loss of steam introduced into subterranean formations, one such attempt being shown in U.S. Pat. No. 3,511,282, issued on May 12, 1970. This patent discloses a dual-wall tube structure having insulation sealed in the annulus between the inner and outer walls by bushings respectively welded at each end between the inner wall and the outer wall. The inner wall is prestressed in tension prior to being welded to the outer wall. The space defined between the inner and outer walls is filled with a conventional insulating material, such as calcium silicate. Although this technique may be satisfactory in some oil field installations, it is not satisfactory for all oil field installations where large temperature differentials are encountered between the inner and outer walls. In this case, even though the inner wall is prestressed in tension, the inner wall as it is heated will elongate with respect to the outer wall so that the inner wall may even change from a tension to a compression condition with the attendant danger of buckling. The magnitudes of the forces generated are such that localized stresses are created in the weld areas causing cracks which permit exposure of the insulation to well fluids and eventually causing failure or degradation of the insulating structure. Centralizers are incorporated to reduce buckling, but may also, in turn, contribute to a loss of heat because of the generally durable nature of such devices.
Another known technique of handling the aforedescribed temperature differential and resulting elongation between the inner and outer walls of an insulating tube is to place a thin walled bellows between the two walls at each end of the assembly, one end of each of the bellows being rigidly attached to the inner wall, and the other end of the bellows being rigidly attached to the outer wall. This technique, of course, relieves the strain on the welds and joining structure between the walls due to the relative movement between the inner and outer walls. However, the bellows introduce other problems, namely, the bellows are comparatively thin walled and delicate, being typically formed from a heat resistant, springly material, which cannot withstand the rough handling normally encountered in the oil patch.